Additive manufacturing processes for producing three dimensional objects are well known. Additive manufacturing processes utilize computer-aided design (CAD) data of an object to build three-dimensional parts. These three-dimensional parts may be formed from liquid resins, powders, or other materials.
A non-limiting example of an additive manufacturing process is stereolithography (SL). Stereolithography is a well-known process for rapidly producing models, prototypes, patterns, and production parts in certain applications. SL uses CAD data of an object wherein the data is transformed into thin cross-sections of a three-dimensional object. The data is loaded into a computer which controls a laser that traces a pattern of a cross section through a liquid radiation curable resin composition contained in a vat, solidifying a thin layer of the resin corresponding to the cross section. The solidified layer is recoated with resin and the laser traces another cross section to harden another layer of resin on top of the previous layer. The process is repeated layer by layer until the three-dimensional object is completed. When initially formed, the three-dimensional object is, in general, not fully cured, and is called a “green model.” Although not required, the green model may be subjected to post-curing to enhance the mechanical properties of the finished part. An example of an SL process is described in U.S. Pat. No. 4,575,330.
There are several types of lasers used in stereo lithography, traditionally ranging from 193 nm to 355 nm in wavelength, although other wavelength variants exist. The use of gas lasers to cure liquid radiation curable resin compositions is well known. The delivery of laser energy in a stereolithography system can be Continuous Wave (CW) or Q-switched pulses. CW lasers provide continuous laser energy and can be used in a high speed scanning process. However, their output power is limited which reduces the amount of curing that occurs during object creation. As a result the finished object will need additional post process curing. In addition, excess heat could be generated at the point of irradiation which may be detrimental to the resin. Further, the use of a laser requires scanning point by point on the resin which can be time-consuming.
Other methods of additive manufacturing utilize lamps or light emitting diodes (LEDs). LEDs are semiconductor devices which utilize the phenomenon of electroluminescence to generate light. At present, LED UV light sources currently emit light at wavelengths between 300 and 475 nm, with 365 nm, 390 nm, and 395 nm being common peak spectral outputs. See textbook, “Light-Emitting Diodes” by E. Fred Schubert, 2nd Edition, © E. Fred Schubert 2006, published by Cambridge University Press.
Many additive manufacturing applications require a freshly-cured part, aka the “green model” to possess high mechanical strength (modulus of elasticity, fracture strength). This property, often referred to as “green strength,” constitutes an important property of the green model and is determined essentially by the nature of the liquid radiation curable resin composition employed in combination with the type of stereolithography apparatus used and degree of exposure provided during part fabrication. Other important properties of a stereolithographic resin composition include a high sensitivity for the radiation employed in the course of curing and a minimum amount of curl or shrinkage deformation, permitting high shape definition of the green model. Of course, not only the green model but also the final cured article should have sufficiently optimized mechanical properties.
Additive manufacturing processes such as stereolithography have long been employed for dental applications. In one common application, a liquid radiation curable resin composition is selectively cured via additive manufacturing processes to produce a plurality of three-dimensional molds, each successive mold representing an incrementally improved realignment position of a patient's teeth. The molds are next post-cured, thermally treated, and cooled. Finally, a thermoplastic sheet is vacuum-formed over the molds to create aligners, which are inserted into a patient's mouth for use in orthodontistry. “Aligners,” or “dental aligners,” as described herein, are also known in the art as “plastic orthodontic appliances,” and are described in, for example, U.S. Pat. No. 8,019,465, assigned to Align Technology, Inc., which is hereby incorporated by reference. The vacuum-forming of dental aligners, which involves the application of considerable force and heat, is described in, for example, U.S. Pat. No. 5,242,304, assigned to Tru-Train, Inc., and is hereby incorporated by reference. From the foregoing, it is imperative that in order to create parts able to withstand the stresses of the dental aligner manufacturing process, the liquid radiation curable resin compositions used to create the molds possess excellent mechanical properties. These mechanical properties can be predicted by measuring the glass transition temperature (Tg) and Young's Modulus.
The finished series of aligners possess visually-imperceptible, but critically different, dimensions to ensure a gradual realignment. Further, the molds must accurately represent the dimensions of a patient's teeth in order to minimize the discomfort associated with wearing an aligner. Therefore, dimensional precision of the initial mold formed via the additive manufacturing process is of paramount importance.
Furthermore, in order to maximize throughput, industry users require that three-dimensional parts must be created via additive manufacturing processes as quickly as possible. This is particularly a requirement in dental applications wherein a multitude of different three-dimensional molds needs to be created for even a single patient to efficiently create a complete set of dental aligners.
Typically, the building process in vat-based additive manufacturing systems is a recurring process consisting of the following repeating steps: 1) the surface of the liquid radiation curable resin composition is exposed to appropriate imaging radiation corresponding to a desired cross-section of the three-dimensional object, thus forming a solid layer; 2) the vertically movable elevator is translated down, further below the surface of the liquid radiation curable resin composition; 3) a mechanical recoater device is translated across the surface of the liquid radiation curable resin composition to assist in forming the next layer of liquid radiation curable resin over the just formed solid layer; and 4) the elevator is translated up such that the distance between the surface of the liquid radiation curable resin and the just formed solid layer of the three-dimensional object is equal to the desired thickness of the layer to be formed.
The speed of the above mentioned process in a vat-based additive manufacturing system is highly affected by the viscosity of the liquid radiation curable resin. Many existing liquid radiation curable resins are highly viscous; that is, they are sufficiently flow-resistant such that they will not readily form a smooth layer of liquid photocurable resin over the just formed solid layer to ensure accurate cure by actinic radiation. With highly viscous resins, forming a new layer of liquid photocurable resin over the top of a previously-cured layer becomes a time consuming process.
First, the viscosity of the resin affects the ability of the liquid radiation curable resin to cover the parts of the just formed solid layer with a fresh, even layer. Consequently, a recoating operation has traditionally been used to simultaneously place and mechanically smooth a fresh layer of resin over a previously cured layer prior to exposure with actinic radiation. In one non-limiting example, this recoating operation has traditionally been performed by means of a “recoating blade.” A recoating blade design is discussed in, for example, Chapman et al., U.S. Pat. No. 5,626,919, assigned to DSM IP Assets, B.V.
Second, the viscosity of the liquid radiation curable resin affects the time it takes for the liquid radiation curable resin to reach equilibrium as a smooth, even surface after the recoating step. Consequently, a programmed “dwell time” has been traditionally used between the end of the recoating operation and the beginning of the exposure of the next layer of resin to appropriate imaging radiation. Both the recoating operation and the dwell time dramatically increase the process time of a typical vat-based additive manufacturing process.
Few resins suited for such vat-based additive manufacturing processes possess a sufficiently-low viscosity to provide uniform coverage and a “self-smoothing” nature. Such resins would obviate the need for a time-consuming recoating operation and dwell time, instead allowing a fresh layer to be placed upon a previously-cured layer by means of a process called “deep dipping.” Deep dipping is an process whereby a fresh layer of liquid radiation curable resin is applied by merely lowering the elevator deeper into a vat of the liquid radiation curable resin at a specified depth. A resin of sufficiently-low viscosity would therefore allow for the liquid to freely and uniformly flow over the previously cured layer without requirement of assistance from a recoating blade or equilibrating dwell-time.
Third, the viscosity of the liquid radiation curable resin also affects the time and difficulty associated with preparing a recently-cured part for post processing operations. In a vat-based additive manufacturing process, upon build completion of a three-dimensional solid part, the solidified portions are removed from the liquid uncured resin. A highly viscous resin will be more difficult to separate from the cured part, wherein a resin of substantially low viscosity will be removed without significant effort. Thus, low viscosity resins reduce the time required to clean a part in order to prepare it for post processing operations.
While there exist few resins possessing sufficiently low viscosity for suitability to many vast-based additive manufacturing applications, fewer still possess the requisite stability to maintain a low viscosity over time. Liquid radiation curable resins for additive manufacturing possess a well-known tendency to increase in viscosity over time upon exposure to ambient conditions, particularly when stored at elevated temperatures. This exacerbates problems associated with high initial viscosities, resulting in increasingly less efficient, more costly, additive manufacturing processes over time. Viscosity stability of the uncured resin is thus also an important property in a liquid radiation curable resin for additive manufacturing.
In a select few of the more demanding applications of additive manufacturing, users require not only long-term physical property stability in the uncured resin itself, but also of the parts created from the liquid radiation curable resin. Fewer still of the most demanding additionally require that liquid radiation cured parts retain maintenance of superior physical properties over months and even years. One such application, in the dental aligner mold manufacturing industry, further requires liquid radiation cured molds created via additive manufacturing to retain both dimensional and physical stability after severe short- and long-term stresses.
Molds for dental aligners created via additive manufacturing with liquid radiation curable resins undergo multiple, intense, short-term post-processing operations prior to their use in contributing to the formation of a dental aligner. In one process, immediately after cure, the parts are removed from a liquid vat of photocurable resin, cleaned, and then placed in a UV post-cure apparatus, often for several hours. A discussion of UV post-curing of parts created via additive manufacturing techniques can be found in, for example, U.S. Pat. No. 5,167,882, assigned to Loctite Corporation, which is hereby incorporated by reference. Additionally, during the fabrication of dental aligner molds, the post-cured part is then heat treated at various times and temperatures. In one example, parts undergo thermal treatment at 100° Celsius for 6 hours; in another example, parts undergo thermal treatment at 140° Celsius for 6 hours. The mold next undergoes a cooling process. Finally, a dental aligner is vacuum formed against the liquid radiation cured, post-cured, heat treated, and cured mold, which is now also subjected to the additional stresses and heat of the vacuum forming process.
Additionally, manufacturers of dental aligners require that molds produced from liquid radiation curable resins be stored long after their initial use in forming an aligner. This is due to the fact that, over the course of treatment, a patient may lose, misplace, or damage one of their original aligners, thereby necessitating a replacement. Retention of the original molds obviates the time-consuming and costly requirement in producing a new mold via a duplicative set of additive manufacturing, post-curing, and heat-treating processes. However, long-term retention is only viable if the original mold maintains dimensional stability, as a replacement aligner will be faithful only to the mold upon which it is vacuum formed.
The environmental stresses placed on original molds created from liquid radiation curable resins are significant over time. Replacement aligners may not be needed for many months or years. Additionally, in an effort to minimize incurring the energy costs associated with climate-controlled warehouses, inventoried parts are often stored in high heat, light, and humidity environments. Exposure to these conditions over long periods of time has a tendency to induce dimensional change in the molds. Selecting a liquid radiation curable resin that ensures dimensional stability of the molds after intense post-processing, and after long-term ambient exposure, is therefore is of unique paramount importance for the dental aligner industry.
In addition to short and long term dimensional stability, the dental aligner industry requires that molds created from liquid radiation curable resins for additive manufacturing possess superior long-term mechanical strength. First, this ensures that the inventoried molds can withstand physical damage during routine handling and storage. Also it is important that the “aged” molds can withstand the heat and forces exerted by the vacuum forming process associated with forming replacement aligners. A known proxy for measuring long-term mechanical strength after exposure to humidity is known as hydrolytic stability. See, e.g., U.S. Pat. No. 7,183,040, assigned to DSM IP Assets, B.V., for a discussion of hydrolytically stable stereolithography resins. Additionally, achieving consistent Young's Modulus values after long term periodic measurement also tends to evidence superior long-term physical durability of a liquid radiation cured part.
In order to achieve the desired balance of properties, different types of resin systems have been proposed. For example, free-radical curable resin systems have been proposed. These systems generally consist of one or more (meth)acrylate compounds (or other free-radically polymerizable organic compounds) along with a free-radical photoinitiator for radical generation. While these systems tend to be low-viscosity and fast-curing, they are known to produce brittle parts that shrink after cure and possess inferior short- and long-term mechanical properties. They are therefore unsuitable for a wide range of stereolithography applications, including use in creating molds for dental aligners.
Another type of resin system potentially suitable for this purpose is a so-called “hybrid cure” type system that comprises (i) epoxy resins or other types of cationically polymerizable compounds; (ii) a cationic polymerization initiator; (iii) acrylate resins or other types of free-radically polymerizable compounds; and (iv) a free radical polymerization initiator.
It is well known in the field of liquid radiation curable resins that hybrid liquid radiation curable resins produce cured three-dimensional articles with the most desirable combination of mechanical properties. A hybrid liquid radiation curable resin is a liquid radiation curable resin that comprises both free radical and cationic polymerizable components and photoinitiators. It is also well known that the cationically polymerizable components of a liquid radiation curable resin primarily contribute to the desirable combination of mechanical properties in a cured three-dimensional article. However, the cationically polymerizable components of a liquid radiation curable resin polymerize at a much slower rate than the free-radically polymerizable components. Consequently, the mechanical properties of the cured three-dimensional article develop over time, long after the initial cure of the hybrid liquid radiation curable resin. Furthermore, many cationically polymerizable components substantially increase the initial viscosity of the radiation curable resin with which they are associated. Also, several cationically polymerizable components are reactive to ambient heat, light, and humidity conditions, and engage in a partial polymerization at unwanted times, thereby increasing the viscosity of the liquid radiation resin with which they are associated over time. The nature and concentration of the various reagents associated with a hybrid cure system is therefore of utmost importance when crafting a resin suitable for specialized applications.
One specific class of hybrid-cure systems suggested for use in stereolithographic resins and other radiation-curable resins include those containing one or more oxetane components. Several references have suggested the use of an oxetane as either a cationically polymerizing organic substance or as a reactive modifier component for such resins, including the following:
U.S. Pat. Nos. 5,434,196 and 5,525,645 (Ohkawa et al.) relate to resin compositions for optical molding which comprises (A) an actinic radiation-curable and cationically polymerizable organic substance and (B) an actinic radiation-sensitive initiator for cationic polymerization.
U.S. Pat. No. 5,674,922 (Igarashi et al.) discusses active energy beam-curable resin compositions which comprise (A) at least one oxetane compound (B) at least one epoxide compound and (C) at least one cationic initiator.
U.S. Pat. No. 5,981,616 (Yamamura et al.) discusses photo curable resin compositions that contain (A) an oxetane compound (B) one or more selected epoxy compounds and (C) a cationic photo-initiator.
U.S. Pat. No. 6,368,769 (Ohkawa et al.) discusses a stereolithographic resin composition that may include mixtures of the following: (A) cationically polymerizable organic substance that could be a mixture of an epoxy compound and an oxetane compound (3-ethyl-3-hydroxy methyloxetane is mentioned as an oxetane compound); (B) selected cationic photo-initiator; (C) radically polymerizable organic substance such as a polyacrylate; (D) radical photo-initiators; and (E) optional organic compounds having two or more hydroxyl groups per molecule (e.g., polyethers).
U.S. Pat. No. 6,413,696 (Pang et al.) discusses liquid, radiation-curable resin compositions that contain (A) 55-90% by weight of at least one solid or liquid actinic radiation-curable and cationically polymerizable organic substance (these may include oxetane compounds, see column 6, lines 42 to 54); (B) 0.05 to 10% by weight of an actinic radiation-sensitive initiator for cationic polymerization; (C) 5% to 25% by weight of an actinic radiation-curable and radical-polymerizable organic substance; (D) 0.02 to 10% by weight of an actinic radiation-sensitive initiator for radical polymerization; and (E) 0.5 to about 40 percent by weight of at least one solid or liquid cationically reactive modifier-flexibilizer, wherein the reactive modifier-flexibilizer is a reactive epoxy modifier, reactive vinylether modifier, reactive oxetane modifier, or mixtures thereof, and wherein the reactive modifier-flexibilizer contains at least one chain extension segment with a molecular weight of at least about 100 and not more than 2,000, wherein component (A) comprises at least one glycidylether of a polyhydric aliphatic, alicyclic or aromatic alcohol having at least three epoxy groups with epoxy equivalent weight between 90 and 800 g/equivalent and at least one solid or liquid alicyclic epoxide with epoxy equivalent weight between 80 and 330 having at least two epoxy groups with a monomer purity of at least about 80% by weight, or mixtures thereof.
European Patent No. 0848294 B (DSM N.V.; Japan Synthetic Rubber Col, LTD. and Japan Fiber Coatings, Ltd.) discusses a process for photo-fabricating a three-dimensional object by selectively curing a photo-curable resin composition comprising an (A) oxetane compound, (B) an epoxy compound and (C) a cationic photo-initiator wherein the oxetane compound (A) is either a compound comprising two or more oxetane rings or a specifically defined oxetane compound.
Japanese Published Patent Application (Kokai) No. 1-0158385 (Asahi Denka Kogyo KK) discusses a resin composition for optically three-dimensional molding containing a cationic polymerizable organic material containing an oxetane ring in its molecule.
U.S. Pat. No. 7,183,040 (Thies et al.) discusses a radiation curable resin composition comprising relative to the total weight of the resin composition (A) 0-29 wt % of a cationically curable component having a linking aliphatic ester group, (B) 10-85 wt % of an epoxygroup containing component other than A, (C) 1-50 wt % of an oxetanegroup containing component, (D) 1-25 wt % of a multifunctional acrylate and a radical photoinitiator and a cationic photoinitiator.
While one or more of the aforementioned references discusses resin compositions optimized for low-viscosity, ideal mechanical properties, or dimensional stability, none provide a resin suited for the full gamut of properties specifically required by the dental aligner manufacturing industry.
Additionally, several references discuss the use of a polyol or hydroxy containing component in a radiation curable resin composition, including the following:
U.S. Pat. No. 6,127,085 (Yamamura et al.) discusses a photo-curable resin composition comprising (A) a specific epoxy compound having a cyclohexane oxide; (B) a cationic photo-initiator; (C) a specific ethylenically unsaturated monomer; (D) a radical photo-initiator; and (E) a polyol.
U.S. Pat. No. 6,136,497 (Melisaris et al.) discusses a method for producing three-dimensional shaped articles with a radiation-curable resin composition containing (A) 20-90% by weight of cationically polymerizing compounds; (B) 0.05-12% by weight of cationic initiator; and (C) 0.5-60% by weight of at least selected cationic reactive modifiers.
U.S. Pat. No. 6,379,866 (Lawton et al.) discusses a photosensitive resin composition comprising (A) 30-70% by weight of a cycloaliphatic diepoxide; (B) 5-35% by weight of an acrylic material selected from aromatic acrylic material or combinations thereof; (C) 10-39% by weight of an aliphatic polycarbonate diol or polytetrahydrofuran polyether polyol; (D) at least one cationic photoinitiator; and (E) at least one free-radical photoinitiator.
U.S. Pat. No. 7,232,850 (Fong et al.) discusses a photocurable resin composition comprising cationically curable compound, an acrylate-containing compound; a hydroxyl-containing compound; a cationic photoinitiator; and a free radical photoinitiator; wherein said resin composition has less than 0.54 equivalents of cationically curable groups, less than 0.10 equivalents of acrylate groups and less than 0.10 equivalents of hydroxyl groups per 100 grams of said resin composition.
U.S. Pat. No. 5,476,748 (Steinmann et al.) discusses an invention of photosensitive resin compositions comprising: (A) from 40 to 80% by weight of at least one liquid epoxy resin having an epoxy functionality of equal to or greater than 2, (B) from 0.1 to 10% by weight of at least one cationic photoinitiator for component (A), (C) from 5 to 40% by weight of at least one liquid cycloaliphatic or aromatic diacrylate, (D) from 0 to 15% by weight of at least one liquid poly(meth-)acrylate having a (meth-)acrylate functionality of greater than 2, the proportion of component (D) constituting a maximum of 50% by weight of the total (meth-)acrylate content, (E) from 0.1 to 10% by weight of at least one radical photoinitiator for component (C) and, where appropriate, (D) and (F) from 5 to 40% by weight of at least one OH-terminated polyether, polyester or polyurethane, which are especially suitable, for example, for the manufacture of photopolymerized layers, especially of three-dimensional objects.
U.S. Pat. No. 8,501,033 (Southwell et al.) discusses a liquid radiation curable resin capable of curing into a solid upon irradiation comprising: (A) from about 0 to about 12 wt % of a cycloaliphatic epoxide having a linking ester group; (B) from about 30 to about 65 wt % of one or more epoxy functional components other than A; (C) from about 10 to about 30 wt % of one or more oxetanes; (D) from, about 1 to about 10 wt % of one or more polyols; (E) from about 2 to about 20 wt % of one or more radically curable (meth)acrylate components; (F) from about 2 to about 12 wt % of one or more impact modifiers; (G) from about 0.1 to about 8 wt % of one or more free radical photoinitiators; and (H) from about 0.1 to about 8 wt % of one or more cationic photoinitiators; wherein the liquid radiation curable resin has a viscosity at 30° C. of from about 600 cps to about 1300 cps.
U.S. No. 2008/0103226 (Xu et al.) discusses a radiation curable resin composition comprising from about 50 wt % to about 70 wt % of a cycloaliphatic diepoxide, from about 5 wt % to about 15 wt % of a polyol, from about 5 wt % to about 15 wt % of an oxetane, from about 10 wt % to about 20 wt % of an aromatic diacrylate, a radical photoinitiator and a cationic photoinitiator. The invention further relates to a process for making a three dimensional article from the resin composition of the invention, the three-dimensional article itself and to the use of the resin composition of the invention.
US 2004/0137368 (Steinmann) discusses a liquid radiation curable resin composition that comprises cationically polymerizable substances, radically polymerizable substances, a hydroxyl functional component and at least one hydroxyl-functional oxetane compound.
The aforementioned references' requirement that a polyol component be used reduces the disclosed resin compositions' usefulness in dental aligner mold applications. Furthermore, none of these references, nor any other known liquid radiation curable resin composition for additive manufacturing, heretofore exists which provides the requisite balance of glass transition temperature (Tg), dimensional precision, viscosity, stability, and long-term mechanical integrity for optimum suitability in dental aligner mold fabrication applications.
The foregoing shows that there is a long-felt, but unmet need to provide a liquid radiation curable resin composition of sufficiently high strength, high precision, low-viscosity, high viscosity stability, high dimensional stability, and high long-term mechanical integrity to be ideally suited for producing three-dimensional molds having properties sufficient for dental aligner applications.